(1) Field of the Invention
The present invention relates to a dispersion compensating technique suitable for a long distance optical transmission system having a large capacity, and particularly to a dispersion compensating method, dispersion compensating apparatus and optical transmission system, which are capable of readily realizing automatic compensation of wavelength dispersion and polarization mode dispersion.
(2) Related Art
Recently, there has been increased transmission speed of optical transmission apparatus such that an optical transmission apparatus of 10 Gb/s has been already practiced and an optical transmission system of 40 Gb/s is being developed. With the increased transmission speed, optical signal waveforms are considerably deteriorated due to wavelength dispersion characteristics of optical fibers and due to polarization mode dispersion characteristics of optical fibers and optical circuits, thereby resulting in a main factor which restricts a transmission distance.
For example, an optical transmission system of 10 Gb/s adopts a dispersion compensation fiber for compensating for wavelength dispersion to thereby allow a long distance transmission over several hundreds of kilometers. In transmitting optical signals at 10 Gb/s over several hundreds of kilometers in such a way, an influence by polarization mode dispersion is less, so that the necessity of polarization mode dispersion compensation is less. Contrary, in transmitting optical signals at 40 Gb/s over several hundreds of kilometers, there can not be neglected: changes in wavelength dispersion characteristics due to temperature changes of optical fiber; and the influence by polarization mode dispersion, thereby requiring a system for automatically compensating for wavelength dispersion and polarization mode dispersion.
As conventional techniques for automatically compensating for wavelength dispersion and polarization mode dispersion, there has been proposed an apparatus comprising separately: a circuit for automatically compensating for wavelength dispersion of optical signals transmitted through an optical transmission path; and another circuit for automatically compensating for polarization mode dispersion of the optical signals; in which a dispersion compensator of each of the circuits comprises a variable dispersion compensating device, a controlling monitor circuit, and a controlling circuit; such as described in Japanese Unexamined Patent Publication No. 7-221705.
Further, as specific conventional techniques concerning wavelength dispersion compensation, there have been known various techniques such as described in: Japanese Unexamined Patent Publication Nos. 8-321805, 9-326755, and 10-276172; the article by G. Ishikawa et al., “DEMONSTRATION OF AUTOMATIC DISPERSION EQUALIZATION IN 40 Gbit/s OTDM TRANSMISSION”, Technical Digest of ECOC'98, pp. 519-520, 1998; the article by Y. Akiyama et al, “AUTOMATIC DISPERSION EQUALIZATION IN 40 Gbit/s TRANSMISSION BY SEAMLESS-SWITCHING BETWEEN MULTIPLE SIGNAL WAVELENGTHS”; Technical Digest of ECOC '99, pp. 1-150-151, 1999. Further, as variable wavelength dispersion compensating devices to be used for wavelength dispersion compensation, there have been known: a VIPA (Virtually-Imaged-Phased-Array) device such as described in the article by M. Shirasaki et al., “Dispersion Compensation Using The Virtually Imaged Phased Array”, APCC/OECC”, '99, pp. 1367-1370, 1999; and an optical device utilizing a FBG (Fiber-Bragg-Grating) reported in the article by M. M. Ohn et al., “Tunable Fiber Grating Dispersion Using a Piezoelectric Stack”, OFC'97, WJ3.
Further, specific conventional techniques concerning polarization mode dispersion compensation are generally classified into an optical processing type, a photoelectric processing type, and an electric processing type, such as described in the article by H. Bulow et al., “Equalization of Bit Distortion Induced by Polarization Mode Dispersion”, Technical Digest of Core and ATM Networks NOC '97, pp. 65-72. Concerning the optical processing type, a multi-staged type is also studied. Both of the optical processing type and the photoelectric processing type require a control of polarization state. Moreover, all of the three processing types require a control of a plurality of controlling terminals. Further, as a controlling method for polarization mode dispersion compensation, there is also known a technique such as described in the article by H. Ooi et al., “Automatic Polarization Mode Dispersion Compensation in 40 Gbit/s Transmission”, IOOC '99, WE5; and the article by D. Sandel et al., “Automatic polarization mode dispersion compensation in 40 Gbit/s optical transmission system”, Electron. Lett., 1998 pp. 2258-2259, which propose a method to control a polarization state by monitoring frequency components being ½ or ¼ of a clock signal included in optical signals after polarization mode dispersion compensation.
However, the aforementioned conventional dispersion compensating techniques involve the following problems:
(1) Although most of the conventional dispersion compensating techniques require a control of polarization state, the polarization state rapidly fluctuates such as due to vibration of optical fibers. This requires rapid polarization control such as on the order of {fraction (1/1000)} seconds.
(2) The controlling algorithm for the plurality of controlling terminals in the conventional polarization mode dispersion compensation is so complicated, particularly in the multi-staged type. Thus, it is difficult to realize such a controlling algorithm simultaneously with the rapid control in the item (1).
(3) Polarization mode dispersion is a complicated phenomenon where optical signal waveforms are distorted, due to closely related phenomena such as: a distribution in the longitudinal direction of main axes of polarization of an optical fiber; variance of a polarization mode dispersion in the longitudinal direction of the optical fiber; fluctuation of polarization of signal light; and polarization mode dispersions in higher orders. Thus, it is difficult to fully compensate for polarization mode dispersion, by those various compensating techniques proposed so far.
(4) Concerning a constitution for separately conducting automatic compensation of wavelength dispersion and automatic compensation of polarization mode dispersion, there has not been yet established a controlling technique for dividing the compensation control of wavelength dispersion and the compensation control of polarization mode dispersion, to realize simultaneously both of the controls.
(5) The automatic compensation of wavelength dispersion and that of polarization mode dispersion are separately conducted, resulting in an increased size of apparatus and an increased cost.
(6) As a monitoring circuit for controlling a variable dispersion compensating device, there are required such as: a circuit for branching output light; a high frequency circuit for monitoring a frequency component, waveform degradation or the like; and a circuit for monitoring a polarization state; which results in an increased size of the monitoring circuit and an increased cost.
(7) The conventional monitoring circuit for wavelength dispersion compensation control is a circuit to monitor a clock signal component included in output light, to conduct a control, relying upon such as a bit rate of an optical signal, a signal format such as NRZ or RZ, and wavelength chirping. Thus, it is difficult to correspond to so-called bit rate free and/or format free systems.